1. Field of the Invention
The present invention relates to high speed impact printers and, more particularly, to impact cushioning and rebound damping apparatus for the print hammers thereof.
2. Description of the Prior Art
Impact printers of the type with which the present invention is primarily concerned effect printing "on-the-fly" at relatively high speeds. More specifically, in one class of such printers, the character dies or type which are arranged to form a given front are mounted in a longitudinal array on a continuously moving carrier which is drawn past an aligned array of selectively actuable print hammers. Interposed therebetween, of course, is the print medium, such as paper in roll stock form, on which characters are to be printed, and a reversibly driven inked (or carbon impregnated) ribbon.
In order to maximize printing speed, it is obvious that the hammers must have a low mass, and be propelled at high speed with minimal friction. To that end, most hammers are of the so-called inertial type, i.e., hammers which upon being subjected respectively and selectively to abrupt impact or driving forces, are thereafter propelled into the print area under their own inertia. While such hammer operation is conducive to high speed printing, it has imposed a serious problem heretofore in regard to effectively damping the kinetic energy imparted return force, as well as any rebound or oscillatory forces, of each print hammer, which forces are often augmented by a spring-biased hammer return force.
Such impact hammer forces if not absorbed or damped in some way, can very readily result in a given print hammer, after reaching a backstop (which defines a retracted non-printing or rest position), rebounding off the backstop in a forward (print) direction with sufficient velocity so as to again impinge against the paper. This, of course, would result in the printing of unintended character images on the paper.
A number of techniques have been employed heretofore to prevent the aforementioned troublesome and detrimental rebounding hammer motion. For example, in one class of printers wherein the magnetic attractive force of core pole faces of an energized solenoid are utilized to effect the "firing" of an associated hammer, a predetermined amount of applied current is maintained in the solenoid coil (or coils) during the rebound return of the hammer to a rest position so as to establish a dynamic type of (opposed field) braking action against the returning hammer. Unfortunately, such prolonged energization of the coil (or coils) substantially doubles the duty cycle of the solenoid, as well as increases the current requirements for the hammer drive mechanism.
In other electromagnetically operated hammer drive systems, wherein the armature of the solenoid, for example, is utilized either directly or indirectly through a coupled actuator to fire the associated hammer, a second independent electromagnet has often been employed heretofore to magnetically attract and hold the rebound-returned hammer against an associated backstop or bumper until the hammer is again intentionally fired. Such an arrangement has the disadvantage, however, of increasing the costs and complexity of the hammer mechanism, and further necessitates a substantial increase in the current requirements of the print hammer drive circuitry.
A more simplified approach to damping hammer rebound forces or oscillations has involved the use of a resilient, shock absorbing type of bumper in combination with one or more recoil springs. Resilient bumpers have also been employed heretofore in conjunction with the aforementioned types of electromagnetic damping systems.
With particular reference to the resilient bumper per se, in one preferred type of impact printer it has taken the form of an elongated bar which passes through a keying slot formed in each of an array of laterally spaced print hammers. The keying slots are dimensioned such that the hammers impact against and are controllably cushioned by the bumper during the firing of each hammer against the paper (to effect the imprinting of a character thereon), as well as after rebounding off of the paper (after the latter has been driven against a type character die on the backside thereof), and upon return to a rest position. A different leaf spring associated with each hammer further augments the rapid return of each hammer from the forward print position to a retracted rest position. Such a hammer-bumper arrangement is disclosed in E. S. Babler, U.S. Pat. No. 3,823,667, issued July 16, 1974, assigned to the assignee of the present invention, and incorporated herein by reference.
The bumper in such a print hammer mechanism has generally been molded out of a suitable elastomer, such as a polyurethane material exhibiting a durometer Shore hardness of about 90A. While such a bumper material has been found to ideally exhibit the desired resiliency or viscous damping effect, i.e., under controlled or laboratory conditions, in practice, this has not always been the case. Rather, elongated resilient bumpers made of the aforementioned material (as well as other known materials exhibiting similarly sought characteristics), have been found to produce unpredictable and inconsistent energy absorbing characteristics along their length.
More specifically, investigation has lead to the realization that an elongated bumper when made of the abovedescribed (or other known) material initially exhibiting the desired energy absorbing characteristics, has had a tendency to stretch or produce a kneading effect not only during the mounting thereof in the print hammer assembly, but during actual printing as a result of the random initial impact and rebound forces exerted thereagainst by the print hammers. Stated another way, elongated resilient bumpers of the type in question, that are common to an array of print hammers, have been found to acquire very detrimental, cross-sectional dimensional variations along their length, albeit not often visible to the naked eye, as a result of both physical handling during assembly, and the random kinetic induced forces exerted thereagainst by the print hammers during normal use.
This, in turn, results in the print hammers either directly or indirectly (through rebound or oscillatory motion) being propelled against a web or paper with randomly different velocities. In the case of rebound driven print hammers, of course, this often results in unintended characters being at least lightly imprinted on the paper. With respect to intended characters to be printed, appreciable variation in print hammer velocity from a desired norm can often lead to characters being printed along a given line with very noticeably different, and often unacceptable, line contrast or definition. Considered another way, character printing with variable line definition can very easily result if the kinetic energy induced forces of the fired print hammers are not uniformly absorbed along the axial length of the resilient body portion of the bumper at a rate which is equatable to the change in velocity of each of the hammers.
Further contributing to the above problems is the fact that extreme variations in temperature can also adversely affect the resiliency or viscous damping characteristics exhibited by a bumper made of a given plastic elastomer. This is particularly true when the printer is exposed to diverse environments such as encountered in many mobile printer applications.